Flash memory stores information in an array of transistors, called “cells,” each of which stores one or more bits (or portions of bits) of information. NOR flash and NAND flash are two current types of flash memory devices. NOR and NAND refer to the type of logic used in the storage cell array. Flash memory is non-volatile, which means that it stores information in a way that does not need power to maintain the stored information.
A flash memory cell includes a control gate (CG), as in other MOS (metal oxide semiconductor) transistors, but also includes a charge storage structure, such as a floating gate (FG) or charge trap (CT), that is insulated by an oxide or other dielectric. The charge storage structure is located between the CG and semiconductor material and stores electrons or holes placed on it. Information is represented by the stored electrons or holes.
NAND array architecture arranges its array of memory cells in a matrix such that the control gates of each memory cell of the array are coupled in rows to access lines, which are conventionally referred to as word lines. The memory cells of the array are coupled together in series, source to drain, between a source line and the data line. Memory cells in a NAND array architecture can be programmed to a desired state. That is, electric charge can be accumulated (e.g., placed) on, or removed from, the floating gate of a memory cell to put the cell into a number of stored states. For example, a single level cell (SLC) can represent two binary states, e.g., 1 or 0. Multi state memory cells, multibit cells, or multilevel cells (MLCs) can store more than two states.
When electrons are trapped on the FG, they modify the threshold voltage (Vt) of the cell. Thus, when the cell is “read” by placing a specific voltage on the CG, electrical current will either flow or not flow between the cell's source and drain connections, depending on the Vt of the cell. This presence or absence of current can be sensed and translated into 1's and 0's, reproducing the stored data.